Homopolymers of TFE are crystalline or partially-crystalline fluoroplastics. Copolymerization with other monomers that introduce side groups into the polymer affects the properties of the polymers. Generally, at low levels of comonomer incorporation, both melting temperature T.sub.m and crystallinity decrease with increasing comonomer content. Crystallinity is reflected, for example, by the heat of fusion per unit mass as determined by differential scanning calorimetry (DSC). At high levels of comonomer incorporation, all detectable crystallinity may vanish. If the monomers and their concentrations are properly chosen, and molecular weight is properly controlled, useful fluoroelastomers or fluoroplastics may be obtained. For example, the TFE copolymer with perfluoro (methyl vinyl) ether (PMVE) in the approximate molar composition TFE/PMVE=70/30 is a well-known fluoroelastomer. On the other hand, copolymers of TFE with HFP or perfluoro (propyl vinyl) ether (PPVE) in the approximate molar compositions TFE/HFP=92/8 and TFE/PPVE=98.5/1.5 are well-known fluoroplastics. These particular perfluoroplastics have nominal T.sub.m of about 260.degree. C. and 305.degree. C., respectively, as judged by the peak of the melting endotherm by DSC. See FIG. 1. The minimum T.sub.m (peak) for fluoroplastics in these compositional families recognized by ASTM standards is 250.degree. C. for TFE/HFP polymers (ASTM D-2116) and 300.degree. C. for TFE/PPVE polymers (ASTM D-3307). Such fluoroplastics have had commercial utility in part because of their relatively high T.sub.m and the mechanical properties that accompany the level of crystallinity associated with relatively low comonomer incorporation.
Partially-crystalline polymers are those which exhibit crystallinity under some conditions, as opposed to those polymers which are non-crystalline under all conditions. A partially-crystalline polymer typically exhibits crystallinity as polymerized, or after fabrication when the melted polymer is cooled slowly enough to permit development of molecular order. As is well-known, quenching from the melt can reduce or even preclude crystalline order in a specimen of a partially-crystalline polymer. The resultant quenched polymer is nevertheless considered partially crystalline because it started out, prior to melting and quenching, in this condition, and this condition is capable of being restored by reheating the polymer and cooling it more slowly. This effect is more likely for polymers with low crystallinity as polymerized. Crystallinity can be detected by several means, including differential scanning calorimetry (DSC).
There has been no interest in, and no commercial presence of, such fluoroplastics with comonomer concentration approaching but not reaching the level that completely eliminates crystallinity. The prior art does not exemplify TFE copolymers with perfluoroolefins having T.sub.m below or even near 200.degree. C.
Certain copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) are known. Bro & Sand t in U.S. Pat. No. 2,946,763 disclose TFE/HFP copolymers with HFP content reflected by a specific infrared ratio, herein called HFP index or HFPI, in the range 1.5 to 6. They use a multiplier of 4.5 to convert HFPI to HFP content in wt %. Bro & Sandt teach that HFPI greater than 6 is extremely difficult to achieve. An HFPI of 4.70 is the highest value exemplified.
Couture et al in U.S. Pat. No. 3,132,124 teach process refinements to make TFE/HFP copolymers with HFPI in the range of 1.5 to 6. An HFPI of 4.0 is the maximum value exemplified.
There are frequent references in the literature to TFE/HFP copolymers having HFP content of from 6.75 to 27 wt %, apparently following the disclosure of the 1.5 to 6 HFPI range and 4.5 multiplier by Bro & Sandt.
Khan & Morgan in U.S. Pat. No. 4,380,618 disclose a process for synthesis of TFE polymers including TFE/HFP and other TFE copolymers. They teach that perfluoroolefin incorporation into TFE copolymers can be up to about 15 mol %. Khan & Morgan use a multiplier of 2.1 to convert HFPI into HFP content in mol %, so that 15 mol % corresponds to an HFPI of 7.14. The preferred compositional range is 6 to 9 mol % (HFPI of 2.86 to 4.29). For the only example of a TFE/HFP polymerization, HFP content in the polymer was 8.54 mol %, corresponding to an HFPI of 4.07.
Concannon in U.S. Pat. No. 4,075,362 refers to TFE/HFP copolymers having up to 95 wt % HFP. Concannon cites only U.S. Pat. No. 2,946,763 as a teaching of a method of making such copolymers. As discussed above, U.S. Pat. No. 2,946,763 teaches that it is extremely difficult to prepare TFE/HFP copolymers having HFPI greater than 6. Consistent with this teaching of extreme difficulty, the highest HFP content of any of the copolymers actually exemplified by Concannon is 25 wt %, corresponding to an HFPI of 5.56 according to the conversion factor of Bro & Sandt.
Fujii et al in U.S. Pat. No. 3,769,252 and Satokawa et al in U.S. Pat. No. 3,904,575 also refer to TFE/HFP copolymers having up to 95 wt % HFP but likewise cite only U.S. Pat. No. 2,946,763 for a teaching of a method of making such copolymers. Therefore, one skilled in the art would not expect the single exemplified TFE/HFP copolymer to have HFPI greater than 6.
Khan in U.S. Pat. No. 4,381,384 discloses continuous polymerization process for TFE polymers including copolymers containing up to 20 mol % of comonomer. For TFE/HFP copolymers, a multiplier of 2.1 was used to convert HFPI to HFP content in mol %, so that the limit of 20 mol % would correspond to an HFPI of 9.5. In the examples for TFE/HFP copolymers, HFP content of 5.4 mol % (HFPI of 2.57) was the highest level actually achieved.
Youlu et al in U.S. Pat. Nos. 4,749,752 and 5,087,680 describe TFE/HFP copolymers having melt viscosity in excess of 10.sup.5 Pa.s and HFP content in the range 12-30 wt %. Although U.S. Pat. No. 5,087,680 claims a TFE/HFP copolymer with HFP content of 12-30 wt %, the main teaching is directed to extra-high-molecular-weight polymer, and the copolymer of the only polymerization example had 14.5 wt % HFP determined by an undisclosed method. The prior-art citation from which one might judge the method of measuring HFP content was to foreign equivalents of U.S. Pat. No. 3,051,683 which discusses the infrared method taught in U.S. Pat. No. 2,946,763, according to which the copolymer having 14.5 wt % HFP exemplified in U.S. Pat No. 5,087,680 would have an HFPI of 3.22. No preferred range for HFP content is stated.
The continuous process of Khan in U.S. Pat. No. 4,381,384 suffers from some disadvantages, including a very high surfactant concentration that must be used to approach stable reactor operation and to enable discharge of the reaction mass without coagulation of the polymer in or in passing through the let-down valve. This high surfactant concentration has adverse economic effect, can make it extremely difficult to isolate the polymer from the dispersion, and can be undesirable in the isolated product.
Khan & Morgan in U.S. Pat. No. 4,380,618 suggest that HFP content in a TFE/HFP copolymer can range up to a level corresponding to an HFPI of 7, approximately, but there is no example of this HFPI level having been actually achieved, nor is there an example of a copolymer having an HFPI of 6, the upper limit taught by Bro & Sand t in U.S. Pat. No. 2,946,763. An obstacle to the achievement of high HFP content in this copolymer is the low reactivity of HFP relative to that of TFE. Khan & Morgan state that higher temperatures may be employed to promote polymerization rate. Couture et al in U.S. Pat. No. 3,132,124 teach that increasing polymerization temperature is beneficial to rate up to about 119.degree. C. However, the advantage to be gained from higher temperatures is limited. A specific problem with high temperature polymerization of low-melting polymer is partial melting of the polymer during the process. This can result, at least, in coating of the reactor wall. A buildup of skin may adversely affect heat transfer, and skin can slough off as undesirable large pieces of polymer.
Khan & Morgan further state that higher pressures may also be used to promote polymerization rate, especially if a comonomer is unreactive relative to TFE. However, this is not always possible, for reasons of safety or equipment design limitations, for example. In Example 1 of Khan & Morgan, HFP partial pressure was 62-70% of total pressure.
Fluoropolymers are known to be useful as processing aids when used in low concentration in other polymers to mitigate effects such as melt fracture or high torque that limit rate of extrusion of these host polymers, thereby improving processibility. Polyolefins are a class of host polymers of particular interest.
Blatz in U.S. Pat. No. 3,125,547 teaches the use of fluoropolymers in polyolefins to facilitate extrusion. A general requirement taught by Blatz is that the fluoropolymer must be molten or fluid at the processing temperature T.sub.p of the polyolefin. Blatz exemplifies fluoroelastomers, which are non-crystalline and for which the glass transition temperature T.sub.g is well below T.sub.p, and, therefore, are fluid at T.sub.p. Blatz also exemplifies tetrafluoroethylene (TFE) homopolymer which melts 55.degree. C. below T.sub.p. There is no teaching about how such fluoropolymers perform when an antiblock agent is present in the host polymer.
Various two-component process aids have been identified in attempts to achieve better performance than that provided by the single component of Blatz. For example, Chapman & Priester in U.S. Pat. No. 4,904,735 teach a synergistic combination of solid and fluid fluoropolymers, e.g., a fluoroelastomer and an unmelted fluoroplastic, to achieve enhanced process aid performance. That invention is said to be applicable to hydrocarbon polymers containing antiblocking agents, but no data are presented. As a further example, Chapman & Priester in U.S. Pat. No. 5,106,911 teach a combination of fluoroelastomer and molten polyvinylidene fluoride fluoroplastic as a processing aid. As shown by Example 1, the average reduction in extrusion pressure achieved by this process aid system was about 14% in the absence of an antiblock agent. Polyvinylidene fluoride contains 3 wt % hydrogen.
A general problem with prior art processing aids is that the benefit of the aid is substantially reduced when an antiblock agent such as silica or talc is added to the polyolefin.
Middleton in U.S. Pat. No. 3,461,129 discloses a solution of a low-melting (83.degree.-145.degree. C.) TFE homopolymer in a fluorinated oxazoline compound. The melting point range cited indicates a very low molecular weight compound and not a high molecular weight polymer.
Barham in U.S. Pat. No. 4,360,488 discloses a process for forming shaped articles from a gel of TFE homopolymer or non-melt-fabricable TFE polymer modified with a low concentration of comonomer. In this process, the polymer is swollen or dissolved in highly fluorinated solvents near the melting point of the polymer, and a working gel forms when the mass is cooled to room temperature. Several perfluorinated cycloalkanes are stated to be suitable solvents for this process.
Yen and Lopatin in U.S. Pat. No. 4,902,456 disclose a process for making a membrane that involves a melt blend of a TFE/HFP copolymer and a chlorotrifluoroethylene oligomer solvent. The melt blends are made at 280.degree.-310.degree. C. and form gels upon cooling, with solvent separation under some circumstances. No prior method of making such microporous membranes by conventional solution immersion casting processes was known because of the inert nature of prior-art TFE/HFP copolymers.
Smith & Gardner [Macromolecules 18, 1222 (1985)] review and discuss dissolution of polytetrafluoroethylene (PTFE). They report that PTFE had been dissolved only in perfluorinated alkanes. In summary discussion, Smith & Gardner point out that many polymers analogous to PTFE and TFE copolymers, i.e., non-hydrogen-bonding polymers, dissolve only just below their melting points.
There are no known disclosures of solutions of melt-processible TFE/HFP copolymers of melt viscosity in the range 0.1.times.10.sup.3 to 10.times.10.sup.3 Pa.s. In particular, there are no known disclosures of such copolymer/solvent systems that are stable and fluid at room temperature.
Thin fluoropolymer films are potentially useful in many ways to provide a surface with fluoropolymer properties on substrates with different properties. Such films or coatings might serve various release (non-stick), frictional, dielectric, protective, or even melt adhesive purposes. One particularly demanding use for such films would be in protecting magnetic data storage media, for which a carbon protective coat and a fluorinated polyether lubricant are now used. This use would take advantage of the inherent properties of a fluoropolymer and could increase the storage density of a magnetic disk, which varies inversely with the square of the distance between the reading head and disk, if the coating could be made thin enough. To be successful, the polymer film would need to be of the order of 10-300 angstroms in thickness.